journal of the korean ceramic society vol. 55, no. 2, pp

Journal of the Korean Ceramic Society

Vol. 55, No. 2, pp. 116~125, 2018.

− 116 −

https://doi.org/10.4191/kcers.2018.55.2.02

†Corresponding author : Myung Chul Chang

E-mail : [email protected]

Tel : +82-63-469-4735 Fax : +82-63-462-6982

Color Variation in Color-shade Polycrystalline Zirconia Ceramics by the Atmosphere Controlled Firing

Myung Chul Chang†

Department of Materials Science and Engineering, Kunsan National University, Gunsan 54150, Korea

(Received September 18, 2017; Revised January 28, 2018; Accepted February 1, 2018)

ABSTRACT

Color shade variation was investigated in zirconia dental blocks, prepared using commercial powders. As a reference color-

shade block we used the color indexes of A2, A3.5, A4 and B3, according to the VITA classical color scale. The zirconia powders

for color shade blocks showed colors of white, yellow, pink and grey, respectively, after firing at 1530oC. The zirconia powders

were mixed according to the recipe of color shade blocks and shaped at lower pressure using a uniaxial hydrostatic press. The

shaped sample was inserted into a vinyl pack and sealed in a vacuum form machine. The shaped block samples were reshaped at

450 bar using an isostatic cold press and fired at 1530oC for three hours. In order to investigate the atmospheric color variation

with firing temperature, the A2, A3.5, A4 and B3 sintered blocks were fired between 700oC and 1300oC under controlled atmo-

sphere of pN2 and pO2. The surface color picture was taken using a smart phone camera and compared with the results obtained

using the VITA classical color scale. Quantitative color index value, CIELAB, was measured using a color-meter. Above 800oC,

the color darkness greatly increased with the increase of the reduction temperature and keeping time.

Key words : Zirconia, Color shade, Dental block, Reduction firing

1. Introduction

ttrium-stabilized zirconia ceramics as dental ceramics1-3)

have been well developed and commercially utilized as

typical dental materials for artificial teeth. In the earlier

stages there were some difficulties in the mechanical stabil-

ity such as stiffness and thermal stability due to the com-

plex chemical situation in oral biology. In 2000, the dental

application of zirconia ceramics was abruptly introduced

and now zirconia ceramics are the best dental materials for

use in veneers, implants, multi-connected bridges, and so

on.4-6) The final issue in dental clinics is always how to

match the color-shade of the artificial veneer surface with

patient tooth color. For color shade-matching, the dentist

asks the dental laboratory to do the work professionally.

Conventionally dental lab artists have tried to reveal three-

dimensional color situations of patient teeth using zirconia

veneer. In recent technology7-10) developments, the materials

process has been based on color shaded zirconia blocks and

the veneer ceramics are obtained through CAD/CAM

machining. Finally, dental lab artists coat the color film on

the surface of the veneer ceramics using coloring glass; the

coated veneer ceramics are then fired at intermediate tem-

perature between 700oC and 800oC. For this purpose, zirco-

nia ceramics such as A2, A3.5, or B3 are specially prepared

and commercially supplied. In this research we show multi-

color shade technology using unique color shaded zirconia

ceramics through atmosphere-controlled firing; our results

may contribute to simplifying the complicated coloring pro-

cess in artificial teeth for dental patients.

2. Experimental Procedure

2.1. Preparation of color shade zirconia dental block

In this experimental process, we prepared color-shaded

zirconia dental blocks A2, A3.5, A4 and B3, which were

based on the VITA Classics color scale [VITA Zhanfabrik H.

Rauter Gmbh & Co, KG D-79713 Germany], commercially

used in dental laboratories. The dental blocks were pre-

pared using commercial zirconia powders such as Zpex-

White [Zpex-W], Zpex-Yellow [Zpex-Y], Zpex-Pink [Zpex-P],

and Zpex-Gray [Zpex-G], which were supplied from TOSOH

Co. in Japan.8,9) We prepared four kinds of color shaded zir-

conia dental blocks using the reported powder formulation

recipes,8,9) shown in Fig. 1. The formulated powders were

hand-mixed in a vinyl packing wrap and the packing wrap

was tumbled using a pulverizer [FRITSCH Spartan, Ger-

many]. Immediately the mixed powders were shaped using

a stainless-steel mold with 12 cm diameter; molded powders

were then pressed for 5 minutes in a hydraulic press under

0.013 ton/cm2. The shaped round sample was inserted into a

vinyl wrap and the wrapped sample was sealed using a vac-

uum packing machine. The sealed sample was hydrostati-

cally shaped at ~ 450 bar using an ICP [Isostatic Cold Press]

machine [ISA-CIP-S30-200, ILSHIN Autoclave, Korea]. The

shaped block was taken out of the sealed package and put

Y

Communication

March 2018 Color Variation in Color-shade Polycrystalline Zirconia Ceramics by the Atmosphere Controlled Firing 117

into the furnace for sintering. In order to make multi-blocks

of A2 and A3.5, the formulated zirconia powders for the A2

block were put into the above stainless-steel mold and tum-

bled using a pulverizer; then, the formulated powders for

the A3.5 block were put on the surface of the A2 powders in

the same mold. The whole mass of powder in each mold was

moved to the uniaxial press. Using the uniaxial press, the

multi-block sample was pressed for 5 minutes in a hydraulic

press under 0.013 ton/cm2. Shaped round samples sealed

with vinyl wrap were hydrostatically shaped using CIP.

The first firing was done at 1100oC for 3 h and then sam-

ples were cooled. Normally calcined blocks are used for

CAD/CAM machining in dental labs. The machined blocks

were finally sintered at 1530oC for 3 h and cooled. The sin-

tered samples showed the corresponding color indexes for

A2, A3.5, A4, and B3, respectively.

2.2. Color variation of color shade zirconia dental

block using atmosphere-controlled firing

As shown in Fig. 2 the color-shaded zirconia blocks were

put in the center region of the tube furnace and fired at

intermediate temperature between 700oC and 1300oC,

according to the firing schedule, under the proper atmo-

sphere mixing of O2 and N2 gas. Critically, the fired samples

showed volumetric color change according to the gas mixing

ratio and the firing temperature schedule, such as heating,

cooling, and holding times. Additionally, we tried to show a

that three-dimensional color change can occur in the sample

body if we can control the temperature schedule of the sam-

ples placed in the center of the longitudinal firing tube, as

shown in Fig. 2. Occasionally we were able to show a two-

dimensional color change along with distance displaced

from the tube center, which means that there is a firing

temperature gradient along with sample length. Addition-

ally, we were able to observe gradual color change along the

depth of the sample block. This finding can lead to novel col-

oring process technology for these color-shade dental zirco-

nia blocks, which can be utilized in dental labs to fabricate

color-shade blocks.

In zirconia ceramics, the white color base is too strong to

reveal the color of added pigments.5) The pigment color dis-

appears5-10) at the required firing temperature above 900oC

because the pigment element cannot be attached to the

white YSZ powders, indicating the hard solid state interac-

tion in 3YSZ powders with color pigment elements such as

FeOx, CoOx and others. No stains or other colorants will

adhere or bond to Zirconia ceramics. In dental laboratories,

it is difficult to reveal the natural tooth color on a zirconia

surface with its stark white color. Normally, artificial teeth

are coated with glass enamel to a thickness between 0.001

mm and 1.0 mm. The color glass is coated on the surface of

the porous zirconia block and samples are fired at 1380oC

for 7 - 8 h. For good quality, this process is performed three

times.

The amounts of FeOx and CoOx in our experimental blocks

were under 1 wt% [Tosoh] and the color changing process

was done through atmosphere firing. The purpose of this

Fig. 1. Preparation process of color shaded dental blocks suchas A2, A3.5, A4 and B3. Inner table shows the batchformulation8,9) for Zpex-W, Zpex-Y, Zpex-P, and Zpex-G, respectively.

Fig. 2. (a) Furnace with the atmosphere control using N2 andO2 gas flow. (b) Atmospheric firing schedule

118 Journal of the Korean Ceramic Society - Myung Chul Chang Vol. 55, No. 2

process is to investigate the color transition due to REDOX

change11,12) of Fe and Co ions in the 3YSZ ceramic matrix.

The color gradation was analyzed using the VITA Classic

Scale and CR-10 PLUS. The solid-state interaction between

the 3YSZ matrix and the coloring elements was analyzed

using Raman spectroscopy.13-16)

2.3. Characterization

2.3.1. Microstructure and solid-state interaction

The microstructure of the atmospherically sintered zirco-

nia block body was investigated and EDS was measured

using Ultra High FE-SEM [Hitachi, SU-8220]. In order to

analyze the color change due to the solid-state incorporation

of Fe, Er, and Co components, Raman spectroscopy

[NTEGRA, NT-MDT Russia] was performed for the fired

samples.

2.3.2. Color shade matching

For color shade matching, color images were taken using

a digital camera (Galaxy A800, Samsung Co.) before and

after annealing. In order to estimate the color of the zirconia

dental blocks, we used the VITA Classical color scale and

measured the Lab color indices using a colorimeter [CR-10-

plus, Minolta]. CIE Lab color indices17,18) were determined

for the spectrum secured through this procedure by using

the color analysis program embedded in the instrument.

The L* value shows brightness, with red ~ green (more red

color for a more negative number and greener color for a

more positive number) in the case of a*, and blue ~ yellow

(bluer color for a more negative number and more yellow

color for a more positive number) in the case of b*. The color

difference (ΔE) was calculated as ΔE = [(ΔL*)2(Δa*)2 (Δb*)2]1/2

using the Lab indices.

CIELAB Color Space used in dentistry

The Commission Internationale de l’Eclairage (CIE)

developed the CIE 1976 L*, a*, b* color space, with the offi-

cial abbreviation CIELAB.17,18) The L* coordinate or axis

quantifies the lightness and has a scale from zero to 100.

The a* axis represents redness-greenness, with zero being

achromatic, a negative a* value representing more green-

ness than redness, and a positive a* value representing

more redness than greenness. The b* axis represents yel-

lowness-blueness, with zero being achromatic, a negative b*

denoting more blueness than yellowness, and a positive b*

denoting more yellowness than blueness. CIELAB was

developed to create a more perceptually uniform color space

that could be correlated with the visual appearance of col-

ors. This color space is currently the most widely used color

space in dental color research as the difference in colors can

be calculated by measuring the Euclidean distance between

two colors and because these color differences have been

correlated with visual perception. Color difference is repre-

sented by the Greek symbol Δ (delta) and is described

according to the following formula,

ΔExy = [(L*x – L*y)2 + (a*x – a*y)

2 + (b*x – b*y)2]1/2

where x and y denote two different colors in the CIELAB

color space and ΔE denotes the overall color difference

between x and y. Due to the mathematical operations in the

formula, total color difference is an absolute value and does

not provide any information about the direction of the dif-

ference. Information about the direction of color difference is

determined by the difference in each of the three CIELAB

coordinates: ΔL*, Δa*, and Δb*, expressed respectively by

the following formulas,

ΔL*xy = L*x – L*y

Δa*xy = a*x – a*y

Δb*xy = b*x – b*y

where x and y denote two different colors in the CIELAB

color space. A disadvantage of CIELAB is that color differ-

ences in the a* and b* axes cannot be described in terms of a

change of hue and chroma, making CIELAB less applicable

in clinical settings.

VITA Classical guide

The VITA Classical (VITA Zahnfabrik, Bad Sackingen,

Germany) shade guide is one of the most commonly used

guides. It has a total of 16 shades that are divided into four

groups based on hue. The manufacturer labels group A red-

dish brown, group B reddish yellow, group C gray, and

group D reddish gray. Shades within each group are based

on changes in Chroma and Value, in which increasing

Chroma and decreasing Value correspond to higher desig-

nated numbers. In principle, shade guides should ade-

quately represent the entire Hue, Value, and Chroma range

of natural teeth. However, this is not the case for the VITA

Classical shade guide when comparing published tooth color

data with published shade guide color data. The VITA Clas-

sical shade guide shows deficiencies in RY Hues: Hues

extends too far Y, range of Value is too narrow, and Chroma

is deficient. How well a guide represents all tooth shades

can also be expressed in terms of coverage error. Coverage

error is the average color difference between the closest

shade guide match and the tooth’s actual/perceived color.

The VITA Classical shade guide is organized into groups

according to hue, with gradations of Chroma and value

within each group.

Shade matching instruments

Colorimeters illuminate a specimen and measure red,

green, and blue light reflected back to the instrument. The

relative amounts of red, green, and blue are termed the tri-

stimulus values, and they define the color of the object. Tri-

stimulus values determined by colorimetry do not generally

agree with those calculated from spectrophotometric data or

those based on the 1931 CIE standard observer and may be

affected by the age of the red, green, or blue filters. Colorim-

eters are commonly used as color difference meters in color

production control, which requires a good precision and a


high accuracy in color difference measurement. The color

evaluation was done using a C-10 PLUS [Minolta, Japan]

colorimeter; L*a*b* value was obtained for the above

zirconia samples. L* indicates Value. L* = 100 is for white

and L* = 0 is for black. a* and b* indicate Hue and Chroma,

respectively. *60 and a*-60 indicate red and green,

respectively. b*60 and b*-60 indicate orange and red,

respectively.

Fig. 3. (a) FE-SEM micrographs for the A2, A3.5, and B3 samples fired in air, and for the A2N, A3.5N, and B3N samples firedin reduction atmosphere under N2 gas flow. (b) FE-SEM-EDS analysis for B3-O1 and B3-3N, respectively.


3. Results and Discussion

3.1. Preparation of color shaded zirconia blocks

The color shaded zirconia blocks showed corresponding

color matches to A2, A3.5, A4, and B3, respectively. Fig. 3(a)

shows the microstructure of the A2, A3.5, and B3 blocks,

which were fired in air atmosphere; the blocks fired in

reduced atmosphere were denoted as A2N, A3.5N, and

B3N, respectively. In Fig. 2, we provide data on the color

variation of the prepared zirconia dental block samples,

Fig. 3. Continued.


which were treated under atmospheric change using N2 and

O2 gas at a higher temperature between 700oC and 1300oC.

Fig. 3(b) shows EDS analysis results for the block samples

in Fig. 3(a). In spite of the great color change, the EDS

results did not show quantitative color element variation

because the color pigment addition was under 1 wt%. The

valence in transition elements can be easily varied with

changes of temperature and atmosphere; a small amount of

abrupt valence transition can yield a corresponding color

change.11,12, 19-21) It is known that zirconia ceramics maintain

a stark white color even at intermediate temperature.5,7)

Above 800oC we were able to observe color variation via

transition metal element addition of such metals as Fe and

Co. In Fig. 4 the color index for the above block samples was

revealed by measuring the CRI values using a Minolta CR-

10-PLUS; the data analysis is explained in section 3-3.

Fig. 4. Lab values for the color-shade block samples of A2, A4, A3.5, and B3, which were again fired in O2 and N2 atmosphere.The Lab values were measured by using Colorimeter. (a) L*a*b* (b) La*-a*-b*


In dental block applications it is important to show the

translucency after final sintering.8,22-24) In order to impart

translucency to zirconia ceramics it is crucial to reduce the

levels of oxygen vacancy and impurity, which cause light

scattering in the grain boundary according to different local-

ized compositions that have different levels of light reflec-

tion. In order to obtain good translucency in 3YSZ dental

ceramics, it has been a key issue24) to deplete the oxygen

vacancies at the grain boundary misfit region, which leads

to an increase of the oxygen concentration. The amount of

alumina as sintering additive has been well studied and

found to increase the sintering density. In the development

of 3YSZ powders for dental blocks, there have been several

technological competitions in the areas of reducing oxygen

pores in the shaping process, controlling the amount of alu-

mina content, and formulating color additives for tooth color

shading.

As a typical coloring agent, the Fe component is homoge-

neously added through wet chemical precipitation in 3YSZ

composition, such as Zpex-Y. The uniform distribution of Fe

component in 3YSZ ceramics is important to obtain a uni-

form color shade. In Zpex-G, the addition of Co component is

being performed using the same process. In order to make

Erbium stabilized zirconia powders, Zpex-P is prepared by

wet chemical method using ZOC [Zirconium Oxychloride]

and Er-chloride compound.7,8)

3.2. Atmospheric color change in the sintered zirco-

nia ceramics

Shade matching in human teeth is subjective and chal-

lenging because color is not easily quantifiable. It is often

difficult to achieve an accurate color match in a dental pros-

thesis for accurate rendition of patient teeth, which have

non-planar surfaces and non-homogenous structure, color,

and translucency.

Color is the visual perceptual sensation of light that

defines the appearance of our surroundings.23,25-29) Light is

the part of the electromagnetic spectrum visible to the

human eye. The visible light range includes wavelengths

from 380 to 760 nm (CIE 1987). Light photons with shorter

wavelengths (400 nm) appear blue and those with longer

wavelengths (700 nm) appear red.

Normally in YSZ ceramics it is known that a mixture of

Fe2O3 and Co2O3 can yield a coloration element for purple.7,8)

Pink is achieved using a polycrystalline zirconia powder sta-

bilized with 8 - 11 wt% Er2O3. All coloring powders with

BET 12 - 13 m2/g were prepared using wet chemical method;

spray-dried powders with 3 wt% binder were supplied by

TOSOH. According to the combination rate (wt.%) in the

Table in Fig. 1, we used the formulation batch8,9) for A2,

A3.5, A4, and B3. A2 had a formulation batch of 57.85 Zpex-

We, 40.0 Zpex-Y, and 2.15 Zpex-P. A3.5 had a formulation

batch of 29.03 Zpex-W, 56.67 Zpex-Y, and 4.30 Zpex-P.

Additionally, we used B3 batches of 53.18 Zpex-W, 43.33

Zpex-Y, 2.15 Zpex-P, and 1.33 Zpex-G.

The atmospheric color change during firing is shown in

Fig. 4; the measured CRI values were analyzed in section 3-

2. The mixing ratio of N2 and O2 was controlled using the

Fig. 5. (a), (b), (c), (d), (e), (f) Optical image for 3YSZ colorceramics samples, which were fired at the scheduledtemperature between 700oC and 1300oC under atmo-spheric gas mixture of N2 and O2.


pressure controller in a gas bomb. The gas flowed into the

plastic tube, and finally was passed through a 100 cc beaker

with a flowing speed of two bubbles per second. Fig. 5(a-f)

shows the color change for the zirconia block samples, which

for PO2 change were atmospherically fired using a mixture

gas of N2 and O2. According to the firing schedule of the

color blocks, it can be seen that the color variation and the

reduction condition, such as the firing temperature and the

keeping time, greatly affect the color darkness above 700oC.

At above 1300oC there is great color darkness. Fig. 5(a)

shows room temperature pictures of A4, B3, and the multi-

block, which were sintered at 1530oC for three hours in air.

In Fig. 5(b, c) the samples, fired again in N2 atmosphere at

1020oC, show a dark color change. The flow rate of N2 gas is

about 300 cc/min. With heating, the N2 gas flow was open at

700oC and maintained from 10 to 120 min at 1020oC. The

sample was cooled to 700oC under N2 flow and to room tem-

perature without N2 gas flow. The color darkness increased

with heating temperature and keeping time. In the multi-

block, the A2 color surface changed to A3 color grade and

the A3.5 color surface changed to B4 after passing through

the B3 grade. The details of the multi-color block are shown

Fig. 6. Raman spectroscopy for the samples prepared in atmosphere control of O2 and N2. (A) B3O and B3N sample. Inner graphshows an enlarged 375.3 cm−1 band spectra. (B) A2O and A2N samples. Inner graph shows an enlarged 376.8 cm−1 bandspectra.


in Fig. 5(d, e). Multi-block was fired at 1300oC in N2 atmo-

sphere. The color change was became dark according to the

reduction of the firing temperature and keeping time. The

A3.5 surface changed to A4 color grade and the A2.0 surface

changed to B4 color after passing through the A3 color

grade. In Fig. 5(f) can be seen the final color grade after

reduction firing at 1300oC. Samples A4 and B3 are refer-

ences for color comparison. Sample surface of A3.5 changed

to dark A4 color.

3.3. Color index measured using color meter

In ZPex dental ceramics, total light transmission can be

attained at 41 - 42% after complete sintering at 1540oC with

heating rate of 600oC/h. The total light transmission at 350 -

600 nm is reduced by the addition of coloring elements such

as Fe, Co, and Er in ZPex-Y, ZPex-G, and ZPex-P, respec-

tively. In ZPex-P, Er is an effective element showing a

bright red, pink color in zirconia ceramics. Normally, in

order to obtain black color in zirconia ceramics, a mixture of

transition metal such as Fe, Co, Mn, and Cr can be utilized,

but it will result in the loss of light translucency because of

the metal oxide mixture, presenting black spots on the zir-

conia ceramics surface. In ZPex-G, black spots can be

observed on the surface of zirconia ceramics due to solid

state interaction of Co element with Fe impurity at sinter-

ing temperature. In this report we checked the color status

in the color shaded zirconia ceramics samples using the

VITA classic color scale and a CR-10PLUS color-meter. In

reliably matching for tooth shades, visual shade matching is

subjective and the results vary among observers and also

for an individual observer. The light source influences shade

matching because the spectral composition of the light

reflecting off an object affects the perceived color as a result

of metamerism.30,31) Shade guides are not uniformly distrib-

uted in the CIELAB color space32) and do not cover the

entire range of natural tooth shades. Finally, enamel trans-

lucency and the polychromatic nature of dentin interact to

produce depth of shade that is difficult to characterize.

3.4. Raman spectroscopy in color-shade block

Figure 6 shows Raman spectroscopy results for the color-

shade zirconia samples. In Fig. 6(a), it is noted that the

Raman band at 834.1 cm−1 shows intensity variation with

the change of oxidation and the 375.3 cm−1 band shows

slight intensity variation. More oxidized samples such as

B3O-1 showed stronger intensity compared to that of the

B3N sample treated under N2 gas. In Fig. 6(b) the more oxi-

dized sample of A2O-1 shows a stronger intensity at a

Raman shift of 376.8 cm−1. The observed Raman spectros-

copy will be further reported through Mössbauer and NMR

[Nuclear Magnetic Resonance] measurement, and molecu-

lar orbital calculation.33)

4. Conclusions

The color variation in 3YSZ dental color-shade ceramics

was investigated by using atmospheric firing using O2 and

N2 gas. The color transition was based on the known mixing

ratio of coloring elements using Fe, Co, and Er in a 3YSZ

matrix. The formulated powders were shaped and sintered

at 1530oC. The sintered bodies were fired again under atmo-

spheric condition of O2 and N2 gas at temperature between

700oC and 1350oC with variation of the firing schedule. The

obtained samples showed various types of color variation

with temperature and firing time. The color was measured

using a Colorimeter. We tried to use EDS to analyze the

optical color variation effect, without success. Additionally,

the measured Raman spectroscopy results showed signifi-

cant data variation. Additional results will be reported

through Mössbauer and NMR measurement, and Molecular

Orbital Calcultion.

Acknowledgments

This research was supported by the general research

support program of the National Research Foundation

(NRF), funded by the Korean Government (NRF-

2017R1D1A1B03032397).

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